Flexible stent structure

ABSTRACT

Luminal prostheses comprise adjacent expansible segments, typically serpentine ring segments joined by sigmoidal links. By properly orienting the sigmoidal links and aligning hinge regions on adjacent serpentine rings, enhanced opening characteristics can be obtained. Additionally, by varying the mechanical characteristics of adjacent serpentine rings, program expansion of the luminal prostheses over their lengths may be obtained. The disclosed prostheses also have controllable opening characteristics and can be crimped to small diameters.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 09/565,560 (Attorney Docket No. 020460-000100/______), filedMay 4, 2000, the full disclosure of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to medical devices and methods.More particularly, the present invention relates to radially expansibleluminal prostheses, such as vascular stents and grafts.

Luminal prostheses are provided for a variety of medical purposes. Forexample, luminal stents can be placed in various body lumens, such asblood vessels, the ureter, urethra, biliary tract, and gastrointestinaltract, for maintaining patency. Luminal stents are particularly usefulfor placement in atherosclerotic sites in blood vessels or fistula orbypass grafts. Luminal grafts can be placed in blood vessels to providesupport in diseased regions, such as aortic abdominal, and otheraneurysms.

Both stent and graft prostheses must meet certain mechanical criteria tofunction successfully. In particular, such prostheses should be at leastpartly flexible or articulated (i.e., adjacent expansible ring segmentsare connected by links that articulate relative to one another) overtheir lengths so that they may be advanced through tortuous body lumens,such as those of the coronary vasculature. In addition, the prosthesesshould have controllable length change properties, either to maintaintheir original length or to have the ability to elongate or foreshorten,as desired, when the prostheses assume an expanded configuration.Further such prostheses must have sufficient mechanical strength,particularly hoop strength after they are expanded, in order tomechanically augment the luminal wall strength and thus maintain lumenpatency. The ability to meet these requirements is severely limited inthe case of stents and grafts which are delivered in a radiallyconstrained or collapsed configuration. Such prostheses must radiallyexpand at a target site within the body lumen, so any adaptations whichare intended to enhance flexibility must not interfere with the abilityto radially expand or to maintain strength once expanded.

Prior luminal prostheses often have structures which present a risk ofinjury as they are endoluminally delivered (i.e., tracked) to and/orreleased at a target site within a patient's body lumen. In particular,many vascular stents comprise a plurality of circumferentially connectedand spaced-apart ring segments which deform circumferentially as thestent is radially expanded. The Palmaz stent described in U.S. Pat. Nos.5,102,417 and 4,776,337, is typical of such stents. Such stent designscan present challenges in both delivery and deployment. For example aphenomenon called “flaring” occurs when the longitudinal elements of thedistal or proximal end of the prosthesis are bent outward to assume acrown-like configuration due to bending forces placed on these elementsas the prosthesis passes through tortuous body passageways. Flaring cancreate the same deleterious effects as the previously described fishscaling phenomenon, injuring or traumatizing the blood vessel wall asthe prosthesis is delivered or tracked within the blood vessel. Inaddition, flaring may increase a tendency for stent movement relative toa delivery balloon, thus causing an improperly deployed stent or,possibly, dislodging the undeployed stent completely from the catheter.

In addition to challenges during delivery, prior luminal prostheses cansuffer problems during expansion, particularly during balloon expansionof malleable stents. For example, it has been found that balloonexpansion of vascular stents often results in the ends of the stentexpanding preferentially compared to the center of the stent. Such“dog-bone” expansion inhibits sufficient expansion of the center or endsof the stent, thus leaving a restricted luminal area in the fullydeployed stent. Conversely, sometimes it will be desired to flare theends of the stent in order to lock the stent in place and prevent theends of the stent from collapsing after deployment. The ability toprogram stent expansion over the length of the stent has generally beenlacking in prior stent designs.

A still further problem experienced by many prior stent designs is alack of vessel coverage after expansion. It will be appreciated that theability to support luminal patency and inhibit hyperplasia and otherluminal in-growth can be enhanced if relative coverage of the luminalwall area by the expanded stent is increased. Thus, stent designs whichafford a greater luminal wall coverage, or which minimize the free spacebetween stent structures, while minimizing the amount of stent materialused may be advantageous. Such increase of luminal wall coverage,however, should not be achieved at the expense of “crimpability.”Particularly for vascular applications, it is desirable that thediameter of the stent be reduced as much as possible during delivery,e.g., when crimped over a delivery balloon. By minimizing thecrimped-stent diameter, both trackability and the ability to crosssmaller lesions and access more distal lesions will be enhanced. Inaddition, a larger crimped-stent diameter may increase the risk of stentmovement relative to the deployment balloon which, in turn, could causean improperly deployed stent or even loss of the undeployed stent fromthe catheter. The ability to reduce the stent diameter is generallylimited by the amount of material in the stent itself. Thus, designswhich increase the ability of the stent to cover the luminal wallwithout significantly reducing the “crimpability” would be particularlydesirable.

For these reasons, it would be desirable to provide improved stent,graft, and other luminal prostheses. In particular, it would bedesirable to provide improved luminal prostheses which exhibit a highdegree of flexibility with minimum losses of hoop strength and luminalwall coverage after the prostheses are expanded. For example, the designshould be such that the expanded prostheses will conform to both curvedand straight vessels with minimal or no straightening or otherunintended deformation of the vessel wall. Such luminal prosthesesshould be trackable, preferably being both flexible and presentingminimum risk of injury to the luminal wall as they are being delivered.In particular, the prostheses should avoid “fish scaling” and should behighly “crimpable” so that the prostheses diameter during delivery canbe reduced. The luminal prostheses will preferably further displaysuperior expansion characteristics. In particular, the prosthesesdesigns should permit selective programming of the expansioncharacteristics along the length of the prostheses. For example, thedesigns should permit preferential expansion over the central portion ofthe prosthesis, or alternatively at either or both ends of theprostheses depending on the particular application in which theprosthesis is to be used. Still further preferably, upon expansion theprostheses should display superior luminal wall coverage and adequate tosuperior hoop strength in order to best maintain patency of the bodylumen being treated. At least some of these objectives will be met bythe luminal prostheses described and claimed hereinafter.

2. Description of the Background Art

Stents having expansible ring segments joined by sigmoidal links andaxial beams are described in WO 99/17680. Stents comprising expansiblerings including struts and hinges where the hinges are configured tohave different opening forces are described in U.S. Pat. No. 5,922,020.EP 662 307 describes an expansible stent having serpentine elements withvarying degrees of curvature to provide controlled expansioncharacteristics. WO 00/003,662 describes a stent delivery balloon whichpreferentially opens a center region of a stent as the balloon isexpanded. U.S. Pat. No. 6,017,365, describes a stent with serpentinesegments with non-linear struts and sigmoidal links. Other patents ofinterest include U.S. Pat. Nos. 4,776,337; 5,102,417; 6,017,362;6,015,429; and 6,013,854.

SUMMARY OF THE INVENTION

The present invention provides improved luminal prostheses suitable forendoluminal placement within body lumens, particularly blood vessels,and most particularly coronary and peripheral arteries. The luminalprostheses may be in the form of stents, intended for maintainingluminal patency, or may be in the form of grafts, intended forprotecting or enhancing the strength of a luminal wall. Generally, theterm “stent” will be used to denote a vascular or other scaffoldstructure comprising expansible components, such as ring segments, whichwhen expanded form an open lattice or framework which is disposedagainst the luminal wall. In contrast, the term “graft” will generallydenote such as luminal scaffold which is covered by a liner, membrane,or other permeable or impermeable layer which covers at least a portionof the scaffold. The drawings included herein are generally directed atstent structures, but it will be appreciated that corresponding graftstructures could be provided by incorporating a liner, membrane, or thelike, on either the outer or inner surfaces of the stent.

The luminal prostheses of the present invention will be radiallyexpansible, usually by the application of a radially outward internalforce to expand a minimally resilient (usually malleable) prosthesisstructure. Such radially outward internal force will usually be providedby an inflatable balloon, and such balloon expansible stents arewell-known in the art and described in the background references whichhave been cited above and are incorporated herein by reference.Alternatively, at least some of the radially expansible luminalprostheses of the present invention may be self-expanding. Byfabricating the prostheses from a resilient material, usually a metal,such as spring stainless steel, a nickel-titanium alloy (such asNitinol® alloy), or the like, the prosthesis can be designed to have alarge (fully expanded) diameter in an unconstrained state. The diameterof the prosthesis can be reduced by applying a radial constraint, e.g.,by placing the prosthesis within a sleeve, tube, or other constrainingstructure. In that way, the self-expanding prosthesis can be deliveredwhile constrained and deployed by releasing the constraint at the targetsite within the body lumen. The general principles of constructingself-expanding stents and other luminal prostheses are also well-knownin the art and described in at least some of the background referenceswhich have previously been incorporated herein.

In a first aspect of the present invention, a radially expansibleluminal prostheses comprises a plurality of serpentine ring segmentsincluding struts connected by hinge regions. The struts may be straightor may have non-linear configurations, e.g., being curved, wavy, or thelike. The use of non-linear struts may be advantageous in order toincrease the area of the strut which engages the luminal wall afterexpansion without significantly reducing flexibility and/or crimpabilityof the strut. The hinge regions are usually formed by a short curved orC-shaped region which permits the connected struts to reverse directionin order to define the serpentine ring pattern. Adjacent serpentinerings are joined by sigmoidal links, i.e., S-shaped elements which maybe malleable or elastically deformable in order to allow the adjacentsegments to flex relative to each other during prosthesis delivery andexpansion. The sigmoidal links are attached to a side of the hingeregion, typically located at the point where the hinge attaches to ortransforms into the strut. The use of such sigmoidal links is beneficialsince it permits the longitudinal expansion or contraction of theprosthesis to accommodate length changes as the prosthesis is expanded.Such links further permit bending of the prosthesis since they allowdifferential motion of adjacent serpentine rings. Such flexibility isparticularly advantageous since it allows improved tracking of theprosthesis as it is delivered to an endoluminal location. The sigmoidallinks also improve the conformability of the expanded prosthesis whenplaced in a native vessel, artificial graft, or other body lumenlocation. Such a structure distinguishes prior art designs where asigmoidal link is attached at or near the apex of the link. By attachingthe sigmoidal link closer to the strut, the adjacent ring segments canbe positioned closer to each other. Moreover, because the links attachaway from the apex of the hinge region, stress at the apex is reducedand uniform expansion of each ring segment is enhanced.

In a second aspect of the present invention, the apexes of opposed hingeregions on adjacent serpentine rings will be circumferentially offset.That is, the hinge regions apices on at least some (often all) of theserpentine rings will be aligned with the trough regions on the adjacentserpentine ring. In this way, the hinge regions are circumferentiallyoffset so that the circumferential length of the sigmoidal linksconnecting proximate hinge regions can be reduced. Such a design alsoallows for adjacent serpentine rings to be closer together to allow forimproved vessel coverage upon prosthesis expansion. Such a design alsopermits an increase in the diameter of the curved portions of thesigmoidal link which further improves stress distribution and openingcharacteristics of the prosthesis. Preferably, the luminal prostheses ofthe present invention will both have the sigmoidal links attached to thesides of the hinge regions and have the hinge regions circumferentiallyoffset in order to achieve the greatest improvement in flexibility,crimpability, and uniform expansion characteristics.

The sigmoidal links will preferably have a S-shaped geometry with twoouter connecting legs joined to a central leg by U-shaped joints. Insome instances, it might also be possible to provide Z-shaped sigmoidallinks, but those will generally be less preferred. In connecting thesigmoidal links to the hinge regions of the serpentine rings, the outerconnecting legs will generally be oriented in the annular orcircumferential direction and attach to the hinge region on its side.

In a further aspect of the present invention, a radially expansibleluminal prosthesis comprises a plurality of ring segments which areexpansible in response to a radially outward force. The ring segmentsmay comprise serpentine rings, as generally described above, or maycomprise zig-zag segments, box segments, or other conventionalprosthesis ring patterns. The expansion characteristics of the luminalprosthesis may be varied over the length of the prostheses bycontrolling or programming the characteristics of each of the adjacentexpansible ring segments. For example, different ring segments can becontrolled to have different cross-sectional areas, e.g., differingwidths, thicknesses or both, so that the amount of radially outwardforce needed to open the stent is lesser or greater. Alternatively, inthe case of serpentine or zig-zag ring patterns, the strut length may bevaried in order to control the force needed to open the stent. That is,ring segments having a greater strut length will open with a lesserforce since the increased strut length will leverage the force appliedto a hinge region so that the hinge region will open sooner. Othertechniques for controlling the expansion characteristics of anindividual ring segment may also be employed, such as those described inU.S. Pat. No. 5,922,020, the full disclosure of which has previouslybeen incorporated herein by reference. Depending on the objective, thering segments near either or both ends of the prosthesis may beprogrammed to open more or less readily so that, when applying aconstant radially outward force along the length of the prosthesis, thestent will first open either at both ends or in the middle. It will beappreciated that by employing balloons which also have variableexpansion characteristics, such as those described in WO 00/03662, awide variety of prosthesis expansion characteristics can be providedover the length of the prosthesis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are digital photographs of a coronary stent constructedin accordance with the principles of the present invention with FIG. 1Ashowing the stent in an unexpanded configuration and FIG. 1B showing thestent in an expanded configuration.

FIG. 2 is a “rolled out” view of a the exemplary scaffold structure ofFIGS. 1A and 1B.

FIGS. 3A through 3C are “rolled out” views of second through fourthexemplary embodiments of scaffold structures which may be programmed todisplay differential expansion characteristics over the length of theprosthesis.

FIG. 4 is a detailed view of the scaffold structure of FIG. 2 showingpreferred dimensions of the scaffold components.

FIG. 4A is a cross-sectional view of a hinge region of a scaffoldstructure of the present invention.

FIG. 5 is a detailed view similar to FIG. 4, showing the optionalincorporation of non-linear struts into the scaffold structure.

FIGS. 6A and 6B are detailed views showing the scaffold structure ofFIG. 2 in its non-expanded configuration (FIG. 6A) and in its fullyexpanded configuration (FIG. 6B).

FIGS. 7A-7C show balloon expansion of a luminal prosthesis, such as thathaving the scaffold structure of FIG. 3A or FIG. 3B, which has beenprogrammed so that it preferentially expands near its center.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The present invention provides luminal prostheses intended forendoluminal placement in body lumens, particularly within the vascularsystem for the treatment of cardiovascular disease, such as vascularstenoses, dissections, aneurysms, and the like. The prostheses, however,are also useful for placement in other body lumens, such as the ureter,urethra, biliary tract, gastrointestinal tract and the like, for thetreatment of other conditions which may benefit from the introduction ofa reinforcing or protective structure within the body lumen.

The prostheses are preferably placed endoluminally. As used herein,“endoluminally” will mean placement through a body opening or bypercutaneous or cutdown procedures, wherein the prosthesis istranslumenally advanced through the body lumen from a remote location toa target site in the lumen. In vascular procedures, the prostheses willtypically be introduced “endovascularly” using a catheter over aguidewire under fluoroscopic guidance. The catheters and guidewires maybe introduced through conventional access sites to the vascular system,such as through the femoral artery, or brachial, subclavian or radialarteries, for access to the coronary arteries.

A luminal prosthesis according to the present invention will usuallycomprise at least two radially expansible, usually cylindrical, ringsegments. Typically, the prostheses will have at least four, and oftenfive, six, seven, eight, ten, or more ring segments. At least some ofthe ring segments will be adjacent to each other but others may beseparated by other non-ring structures.

By “radially expansible,” it is meant that the segment can be convertedfrom a small diameter configuration (used for endoluminal placement) toa radially expanded, usually cylindrical, configuration which isachieved when the prosthesis is implanted at the desired target site.The prosthesis may be minimally resilient, e.g., malleable, thusrequiring the application of an internal force to expand and set it atthe target site. Typically, the expansive force can be provided by aballoon, such as the balloon of an angioplasty catheter for vascularprocedures. As will be described below, the present invention preferablyprovides sigmoidal links between successive unit segments which areparticularly useful to enhance flexibility and crimpability of theprosthesis.

Alternatively, the prosthesis can be self-expanding. Such self-expandingstructures are provided by utilizing a resilient material, such as atempered stainless steel or a superelastic alloy such as a Nitinol®alloy, and forming the body segment so that it possesses its desired,radially-expanded diameter when it is unconstrained, i.e. released fromthe radially constraining forces of a sheath. In order to remainanchored in the body lumen, the prosthesis will remain partiallyconstrained by the lumen. The self-expanding prosthesis can be trackedand delivered in its radially constrained configuration, e.g., byplacing the prosthesis within a delivery sheath or tube and removing thesheath at the target site.

The dimensions of the luminal prosthesis will depend on its intendeduse. Typically, the prosthesis will have a length in the range fromabout 5 mm to 100 mm, usually being from about 8 mm to 50 mm, forvascular applications. The small (radially collapsed) diameter ofcylindrical prostheses will usually be in the range from about 0.5 mm to10 mm, more usually being in the range from 0.8 mm to 1.25 mm forvascular applications. The expanded diameter will usually be in therange from about 1.5 mm to 50 mm, preferably being in the range fromabout 2.5 mm to 30 mm for vascular applications.

The ring segments may be formed from conventional materials used forbody lumen stents and grafts, typically being formed from malleablemetals, such as 300 series stainless steel, or from resilient metals,such as superelastic and shape memory alloys, e.g., Nitinol® alloys,spring stainless steels, and the like. It is possible that the bodysegments could be formed from combinations of these metals, orcombinations of these types of metals and other non-metallic materials.Additional structures for the body or unit segments of the presentinvention are illustrated in U.S. Pat. Nos. 5,195,417; 5,102,417; and4,776,337, the full disclosures of which are incorporated herein byreference.

Referring now to FIGS. 1A and 1B, an exemplary luminal prosthesis 10particularly intended for implantation in the coronary vasculaturecomprises from 4 to 50 ring segments 12 (with 7 being illustrated). Eachring segment 12 is joined to the adjacent ring segment by at least oneof sigmoidal links 14 (with three being illustrated). Each ring segment12 includes a plurality, e.g., six, strut/hinge units (described in moredetail in connection with FIGS. 2-5 below), and two out of each sixhinge/strut structures on each ring segment 12 will be joined by thesigmoidal links 14 to the adjacent ring segment. FIG. 1A shows theprosthesis 10 in a collapsed or narrow diameter configuration while FIG.1B shows the prosthesis in its expanded configuration.

Referring now to FIGS. 2 and 4, a first embodiment of a luminalprosthesis 20 constructed in accordance with the principles of thepresent invention will be described in detail. The prosthesis 20comprises serpentine ring segments 22, where each ring segment hasessentially identical characteristics. The ring segments 22 comprise aplurality of linear struts 24 joined by curved hinge regions 26 and 26a. As illustrated also in FIG. 4, the hinge regions 26 are free fromother structure, i.e., they are not linked to adjacent hinge regions orother prosthesis structure. The hinge regions 26 a, in contrast, areconnected or joined to sigmoidal links 28 which secure the adjacent ringsegments 22. In the embodiment of FIG. 2, each adjacent serpentine ringsegment 22 is joined by three sigmoidal links 28. The number of lengths,however, could vary from one, two, up to the total number of hingeregions, i.e., six in the illustrated embodiment of FIG. 2.

The sigmoidal links 28 are adjoined to the hinge regions 26 a so that afirst outer leg segment 30 connects to the hinge region at its basei.e., where the hinge opens into the strut 24. Similarly, a second outerleg segment 32 is joined to hinge region 26 a on the adjacent serpentinering 22 at the base of that hinge region. The legs 30 and 32 aregenerally oriented in a circumferential or annular direction at thepoint where they attach to the hinge regions 26 a. The legs are joinedby a pair of U-shaped regions which join a central leg 34 to completethe sigmoidal link. This design of the sigmoidal link has a number ofadvantages. For example, by orienting the leg segments 30 and 32circumferentially, the legs can move circumferentially past each otherto accommodate radial crimping of the prosthesis as well as facilitateradial opening of the stent. Additionally, the structure permits axiallyshortening and elongation to permit bending of the prosthesis as it isbeing introduced through tortuous regions of a blood vessel or otherbody lumen.

As also best seen in FIG. 4, the serpentine ring segments 22 arerotationally oriented relative to each other so that the apices on thehinge regions 26 and 26 a on the first ring segment are aligned with atrough region 34 on the adjacent ring segment. Such relative rotationalalignment of the ring segments 22 minimizes the circumferential lengthof the sigmoidal links 28 need to connect the opposed hinge regions. Itwill be appreciated that if the apices of opposed hinge regions 26 and26 a were rotationally aligned, the length of the connecting leg 34would have to be significantly longer. Minimizing the length permitsoptimum configuration of the sigmoidal link. By minimizing the length ofthe connecting legs of the sigmoidal link, the crimped diameter can beminimized as the sigmoidal links will not be interfering with thecrimped configuration. In addition, by having shorter connecting legs onthe sigmoidal link, the gaps between adjacent ring segments on theexpanded prosthesis will be minimized. Preferably, leg segments 30 and32 each have flared ends 31 and 33, respectively, which connect thesigmoidal link 28 to the adjoining hinge region 26 a. The flared endsprovide stress relief as the ring segments 22 and links 28 are expanded.

The dimensions of the hinges, struts, sigmoidal links, and the like, ofthe luminal prostheses of the present invention may vary considerablydepending on the intended use. Exemplary dimensions intended for acoronary stent (FIG. 4) are set forth in Table I below. TABLE IExemplary Dimensions (mm) A B C Broad Range 0.025 to 1.25   0 to 6.50.075 to 1.25  Preferred Range 0.075 to 0.15 0.15 to 0.25 0.2 to 0.4 D EF Broad Range  0.1 to 6.5   0 to 6.5 0.025 to 0.65  Preferred Range 0.25to 0.5 0.15 to 0.3  0.035 to 0.075

Referring now to FIGS. 3A through 3C, embodiments of the luminalprostheses of the present invention where adjacent serpentine ringsegments have different expansion characteristics will be described. InFIG. 3A, a luminal prosthesis 30 has a plurality of adjacent serpentinering segments 32 a-32 m. Individual serpentine ring segments 32 a-32 mwill have different expansibility characteristics so that the prosthesis30 will differentially open along its length in response to uniformopening forces. The expansibility characteristics of the individual ringsegments may be modified in a number of the different ways. In a firstgeneral approach, the yield profile(s) of some or all of the hinges inan individual ring segment can be modified relative to such profiles forothers of the ring segments. This can be done by increasing ordecreasing the width, thickness, or other cross-sectional dimension ofany one or more of the hinge regions. Alternatively, the lengths of thestruts connecting the hinge regions can be increased or decreased tochange the leveraged force applied to a hinge region when the prosthesisis being expanded by an internal radially outward force. Otherapproaches, such as adjusting the radius of an arcuate hinge region arealso known.

FIG. 4A is a cross-sectional view of the hinge region 26 or 26 a havinga width W and a thickness T. A hinge region 26 or 26 a having a smallerwidth W, and/or a smaller thickness T, will have less resistance toopening than a hinge region which has a larger thickness T₂ and/or asmall width W₂. Thus, as described above, the opening characteristics ofthe prosthesis can be programmed by adjusting the widths and/orthickness of the hinge regions in particular ring segments 22.

In the particular embodiment of FIG. 3A, the serpentine ring segments 32a-32 c and 32 k-32 m have struts and hinge regions with a larger widththan those of the middle ring segments of 32 d-32 j. The innermost ringsegments 32 f-32 h have the smallest widths for the struts and hingeregions. Thus, assuming the prosthesis 30 is formed from the samematerial (or different materials having the same mechanical properties)over its entire length, the hinges of the ring segments 32 havinggreater widths will be stiffer and provide more resistant to expansion.In contrast, the ring segments having the narrowest widths (32 f-32 h)will be the least stiff and have the least resistance to expansion inresponse to an internally applied force. Thus, when the prosthesis 30 isexpanded over a balloon 42 on a balloon catheter 40, the prosthesis willpreferentially expand over the central region, as illustrated in FIG.7A-7C. In particular, the prosthesis 30 is shown in FIG. 7A prior toexpansion. As the balloon 42 is partially expanded (FIG. 7B), theprosthesis 30 begins opening in its middle sections prior to its endsections. Finally, after the balloon is fully expanded (FIG. 7C),uniform expansion of the prosthesis 30 along its length can be achieved.Of course, when expanded in a body lumen, full expansion of theprosthesis 30 may be constrained so that the center sections will firstengage the wall with the end sections engaging the wall at a later time.Such deployment may be advantageous since it assures that the centralregions of the prosthesis are fully engaged against the luminal wallprior to opening of the end portions.

Under other circumstances, however, it may be desired to preferentiallyopen the end portions of the luminal prosthesis first. In suchinstances, the luminal prosthesis 30 could be modified so that the endsegments 32 a-32 c and 32 k-32 m open preferentially with respect to thecentral ring segments 32 d-32 j. A variety of other openingcharacteristics, such as tapered would also be possible. For example, atapered opening could be achieved by providing a stiffness gradientwhere segments at one end, such as 32 a, are the least stiff with ringsegments becoming progressively stiffer in the direction of ring segment32 m.

Differential expansion of different ring segments can be achieved in avariety of ways. For example, as shown in FIG. 3B, instead of selectingdifferent widths or other cross-sectional dimensions for the hingeregions of the ring segment, the strut length could be varied. As shownin FIG. 3B, the struts in end segments 50 a and 50 f are the longest,with the strut lengths in the inner ring segments 50 b-50 e becomingprogressively shorter. Ring segments having longer strut lengths willapply a greater force to the hinge regions of those struts in responseto an equal radially outward expansion force. Thus, as shown in FIG. 3B,the end ring segments 50 a and 50 f will preferentially open withrespect to the inner ring segments 50 b-50 e. Still further ways forcontrolling the expansion characteristics of an individual ring segmentcould also be utilized. For example, the hinge regions could be weakenedand/or strengthened, as described in detail in U.S. Pat. No. 5,922,020.Alternatively, the diameters of the hinge regions could be varied insuccessive ring segments, as described generally in European patentapplication 662 307.

Referring now to FIG. 3C, serpentine ring segments 60 can be programmedto have different expansion rates by adjusting the lengths of some, butnot all, of the struts 64 in any ring. The serpentine rings 60 will bejoined by sigmoidal links 62, generally as described above. Rather thanhaving struts 64 with identical lengths, those struts which terminate inhinges 66 a which are not adjacent to the sigmoidal links 62 can be madelonger than those struts 66 b which are adjacent the sigmoidal links.This ability offers an additional degree of freedom in programming theexpansion rates of the individual serpentine rings as well as theoverall prosthesis made from such rings.

In an optional aspect of the present invention, at least some of theserpentine ring segments may employ non-linear struts. As shown in FIG.5, ring segments 12 may comprise non-linear struts 50 which are joinedby sigmoidal links 52 in a manner described above in connection withother embodiments of the present invention. The use of non-linear strutsis advantageous in that it increases the length and therefore the amountof strut available for engagement against a luminal wall withoutincreasing the length of the prosthesis. The ability to increase thecoverage of the stent against the luminal wall is well recognized in theart. In addition, the use of non-linear struts can reduce thecrimped-stent diameter since the wall thickness of the stent itself canbe decreased without loss of expanded hoop strength. Another advantageof a non-linear strut design is that it increases the amount of materialwithin the strut thus improving upon the fluoroscopy characteristics ofthe stent.

Referring now to FIGS. 6A and 6B, the improved expansion characteristicsof the luminal prosthesis 20 of FIG. 2 will be described. It is assumedthat the prosthesis 20 has been placed over a delivery balloon and thata constant expansion force over the length of the prosthesis is beingapplied. FIGS. 6A and 6B show a detailed section of two adjacentserpentine rings 22, with the prosthesis shown in its fully collapsed orcrimped condition in FIG. 6A and its fully expanded condition in FIG.6B. As pressure is applied to the delivery balloon, an outward radialforce is applied to the prosthesis. This radial force causes the hingeregions of the serpentine rings to flex open as the prosthesis isexpanded. For a material such as stainless steel, the stresses withinthe hinge region become higher than the yield strength and enter theplastic region of the material. This allows the prosthesis to remainopen following removal of the delivery balloon. For a shape-memory orother resilient alloy, such as Nitinol®, the natural state of theprosthesis is in the expanded configuration. For such “self-expanding”designs, a sheath is holding the prosthesis in a crimped configurationfor delivery to the lesion site. Once the desired deployment locationhas been reached, the sheath is retracted and the prosthesis is expandedto its natural position. The hinges act as springs in causing suchexpansion.

While the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications, and equivalentsmay be used. Therefore, the above description should not be taken aslimiting the scope of the invention which is defined by the appendedclaims.

1. A radially expansible luminal prosthesis including a scaffoldcomprising: a plurality of serpentine ring segments including strutsconnected by circumferentially offset hinge regions, each hinge regionhaving an apex and side regions disposed on either side of the of theapex; and links between at least some of the hinge regions on adjacentserpentine rings, wherein the links are attached on the sides of thehinge regions, and all links have substantially similar sigmoidalshapes.
 2. A radially expansible luminal prosthesis as in claim 1,wherein each sigmoidal link comprises two connecting legs and whereineach connecting leg is attached to a hinge region in a circumferentialdirection.
 3. A radially expansible luminal prosthesis as in claim 2,wherein each sigmoidal link has a uniform width over its length.
 4. Aradially expansible luminal prosthesis as in claim 2, wherein eachsigmoidal link has a flared end with joins to the hinge region.
 5. Aradially expansible luminal prosthesis as in claim 2, wherein thesigmoidal link is adapted to permit the connecting legs to movecircumferentially past each other to accommodate radial crimping of theprosthesis.
 6. A radially expansible luminal prosthesis as in claim 1,wherein the ring segments are expansible in response to a radiallyoutward force; wherein at least one of the ring segments opens at adifferent rate or in a different amount than at least one other ringsegment when exposed to the same radially outward force.
 7. A radiallyexpansible luminal prosthesis as in claim 6, wherein at least somestruts in at least some of the serpentine rings have different lengthsto cause a different rate or amount of expansion.
 8. A radiallyexpansible luminal prosthesis as in claim 7, wherein at least somestruts which are positioned away from the sigmoidal links are longer. 9.A radially expansible luminal prosthesis as in claim 6, wherein at leastsome of the struts have different widths than others of the struts tocause a different rate or amount of expansion.
 10. A radially expansibleluminal prosthesis as in claim 6, wherein at least some of the hingeregions have different widths than others of the hinges regions to causea different rate or amount of expansion.
 11. A radially expansibleluminal prosthesis as in claim 1, wherein at least some of the strutsare straight over the distance between the hinges.
 12. A radiallyexpansible luminal prosthesis as in claim 1, wherein at least some ofthe struts are non-linear.
 13. A radially expansible luminal prosthesisas in claim 1, wherein the sigmoidal links are individually axiallyexpansible and contractable.
 14. A radially expansible luminalprosthesis including a scaffold comprising: a plurality of serpentinering segments including struts connected by hinge regions beingcircumferentially offset with apices and side regions disposed on eitherside of the apices; and links extending between and connecting at leastsome of the hinge regions on adjacent serpentine rings, wherein eachlink is attached on a side of each connected hinge region and whereinall the links have substantially similar sigmoidal shapes.
 15. Aradially expansible luminal prosthesis as in claim 14, wherein theconnected hinge regions on adjacent serpentine rings are axiallyproximate one another.
 16. A radially expansible luminal prosthesis asin claim 14, wherein less than all of the hinge regions of a serpentinering are connected to hinge regions of an adjacent serpentine ring. 17.A radially expansible luminal prosthesis as in claim 14, wherein all ofthe hinge regions of a serpentine ring are connected to hinge regions ofan adjacent serpentine ring.
 18. A radially expansible luminalprosthesis as in claim 14, wherein a number of the connected hingeregions of the serpentine rings is constant along the length of theprosthesis.
 19. A radially expansible luminal prosthesis as in claim 14,wherein a number of the connected hinge regions of the serpentine ringsvaries along the length of the prosthesis.
 20. A radially expansibleluminal prosthesis including a scaffold comprising: a plurality ofserpentine ring segments including struts connected by hinge regionshaving apices and side regions, wherein the side regions are disposed oneither side of the of the apices; and links between at least some of thehinge regions on adjacent serpentine rings, wherein the links areattached on the sides of the hinge regions and all links havesubstantially similar sigmoidal shapes.
 21. A radially expansibleluminal prosthesis as in claim 20, wherein opposed hinge regions onadjacent serpentine rings are circumferentially offset.
 22. A radiallyexpansible luminal prosthesis including a scaffold comprising: aplurality of serpentine ring segments including struts and hinge regionsdisposed between the struts, each hinge region having an apex and sideregions disposed on either side of the apex; links extending from one ofthe side regions of at least some of the hinge regions and connecting atleast some of the adjacent serpentine rings, wherein all links havesubstantially similar sigmoidal shapes; and cells formed by a boundarydefined by adjacently joined serpentine ring segments and a pair ofradially adjacent links connecting the adjacent serpentine rings,wherein each cell has a shape which is a multiple of the smallest cell.23. A radially expansible luminal prosthesis as in claim 22, whereineach cell has a substantially similar shape.